Embodiments generally relate to methods and systems for estimating defibrillation thresholds (DFTs), and more particularly to estimating DFTs based on an upper limit of vulnerability.
Numerous devices and systems exist today that monitor and treat abnormal behavior of the heart (arrhythmias). Examples of arrhythmias include tachycardia, ventricular fibrillation, atrial fibrillation and the like. In general, a device that stops tachycardia and fibrillation of the heart does so by delivering an electrical counter shock to all or a portion of the heart through one or more leads implanted within or proximate to the outer surface of the heart. The electrical shock delivered by the device depolarizes the heart in order to break an abnormal rhythmic cycle and provide an opportunity for the heart to reestablish a normal sinus rhythm. To be successful, the device must deliver an electrical shock that is sufficiently strong to exceed a defibrillation threshold.
When an implantable medical device and the associated lead or leads are implanted, a protocol is followed to determine a defibrillation threshold (DFT) for the patient. The DFT represents the amount of energy needed to effectively defibrillate or cease a tachycardia event and is dependent in part on the implanted lead configuration, the placement of the implanted leads, the individual heart responsiveness to electrical counter shocks and the like.
Methods and systems have been proposed in the past to determine the DFT. One prior DFT determination protocol attempts to predict the defibrillation threshold energy based on a determination of the upper limit of vulnerability (ULV) level for the patient. The ULV level represents an energy strength at which ventricular fibrillation is not induced when the electrical shock is delivered during the vulnerable phase of the cardiac cycle. The vulnerable phase generally is at a time interval near the T wave.
The conventional protocol searches for the ULV level by delivering ULV shocks at various times during the period of vulnerability relative to the T wave. The ULV shocks are delivered at predetermined time intervals prior to or after the peak of the T wave. If the first ULV shock fails to induce ventricular fibrillation, the next test shock may be delivered during a subsequent cardiac cycle at the same strength, but at a different interval relative to the peak of the T wave. This process is repeated for a desired number of time intervals relative to the peak of the T wave. If ventricular fibrillation is not induced after any of the desired ULV shocks, the shock strength is decreased by a set amount and a new group of ULV shocks are delivered in successive cardiac cycles at the predetermined time intervals relative to the peak of the T wave. This process is repeated until fibrillation is induced one or more times. The upper limit of vulnerability represents the ULV level at which fibrillation was induced by a shock delivered during the period of vulnerability.
Conventional protocols that estimate DFTs based on the ULV level have experienced certain limitations. First ULV based estimates generally require inducement of one or more fibrillation events which is not desirable. Also, the ULV level may not be near the DFT, particularly for patients with high DFTs.
Generally, a relation exists between the ULV and the DFT and the associated variables (e.g., shock timing relative to the T wave, shock wave form, electrode configuration, pacing beat and intrinsic conduction). In many patients, the ULV level may closely correlate with the DFT. However, in certain patients, the ULV level may differ significantly from the DFT, such as by 20 Joules and the like. In patients where the ULV and DFT significantly differ, such patients exhibited very unhealthy myocardial characteristics, such as having very low ejection fractions, dilated left ventricles and/or left ventricular hypertrophy. Conventional ULV based DFT estimates have not taken into consideration electrical field distribution and local conduction information.
Generally, fibrillation may be terminated when a majority of the myosites (e.g., a critical mass) of the heart are synchronized by a stimulus. Each stimulus propagates across the heart and exhibits a particular electrical field distribution. The electrical field distribution may be characterized in part by local conduction information, such as the local voltage gradients exhibited at different regions of the heart. The voltage gradient may be characterized as a voltage difference per unit distance across a region of the heart. Different regions of the heart exhibit different voltage gradients. When a shock is delivered from an RV lead through electrodes in the right ventricle, the region of the heart wall proximate to the RV electrode will generally exhibit a higher voltage gradient, as compared to regions of the heart wall that are more remote from the RV electrode. Thus for example, when an RV tip, ring or coil electrode deliver a high energy shock, the heart wall region surrounding the right ventricle that is proximate to the shocking electrode will exhibit high voltage gradients; whereas the right atrial wall, left ventricular wall and left atrial wall will exhibit different voltage gradients that are typically less than the voltage gradients experienced across the right ventricular wall.
Different field distributions are created by different lead configurations, therapy sites and pacing/shocking schemes. For example, when a pacing pulse is delivered near the apex of the RV, one field distribution of voltage gradients will result. When a pacing pulse is delivered near the AV node or near the SVC, second and third different field distributions of voltage gradients will result. The field distribution affects the starting region of depolarization, progression of depolarization and the ending region of depolarization.
Differences in the timing of repolarization for ventricular cells in different regions of the heart depend on the pacing sites and intrinsic beats that occur. During sinus beats in a normal heart, depolarization waves spread in the Purkinje system rapidly and the posterior LV free wall depolarizes last within a cardiac cycle, thereby being marked as the S wave within an ECG signal. When abnormal conduction exists, such as when a left bundle branch block (LBBB) occurs or when an infarction in the left ventricle exists, the time delay of depolarization in the left ventricle increases. The region depolarized last within the cardiac cycle will repolarize last. Similarly, the region depolarized first in the cardiac cycle will repolarize first. The locations of the regions that depolarize and repolarize first and last in the cardiac cycle vary based on, among other things, the pacing sites.
As noted above, at least one conventional protocol has been provided that finds the ULV by shocking at various test intervals based on the peak of the T wave and at predetermined stepped energy levels. The conventional protocol, during the ULV testing, pacing pulses are delivered by an RV or RA electrode. The paced pulses are delivered in the same region of the heart where a subsequent ULV shock is delivered. The field distribution, that is initiated by an RV or RA pacing pulse, propagates from the RV or RA electrode outward across the heart. This conventional protocol has certain limitations. Delivering paced pulses in the same region where the ULV shocks are delivered may not yield a desired repolarization timing sequence. For example, in the conventional protocol, when the patient exhibits slow conduction or left bundle branch block to the LV, the LV free wall will repolarize last. During a ULV test shock delivered relative to the peak of the T wave, part of the ventricular cells in the LV free wall will still be in the refractory state. If the LV free wall is in a refractory state when a ULV test shock is delivered, the heart may be less vulnerable to the ULV shock and thus not enter a fibrillation state. This will lead to an unduly low ULV that is determined, much lower that the DFT in patients that exhibit dilated left ventricles or left ventricular hypertrophy or slow conduction due to some other myocardial conditions.
Embodiments are described herein that seek to provide new and more reliable methods and systems for determining the ULV and in turn, better estimate the DFT which will reduce ventricular fibrillation induction and simplify implantation procedures. Embodiments are described herein that seek to provide new methods and systems that afford a first order estimation of the DFT without the need to induce any arrhythmia.